…Crash

This CL early returns when `_startDownloadBlock` is null. This issue is
currently causing crashes in M125. The workaround was added until we
find the root cause.

Bug: 339626229
Change-Id: I8f4c380995533daf0a40c6d635a3da1e9fbf0ab2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5536265
Reviewed-by: Krishna Govind <govind@chromium.org>
Owners-Override: Krishna Govind <govind@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/branch-heads/6477@{#8}
Cr-Branched-From: 593a01a49dff324f5af5192100d4524a64271b2f-refs/heads/main@{#1299989}